Signal isolators, or isolators, as those terms are used herein, are devices that accept an electrical signal via an isolator input port and provide a replica of that signal at an isolator output port, while providing galvanic isolation between the input and output ports. Signal isolators may be used for safety, e.g., to isolate selected portions of a circuit from high voltages in another part of a circuit, and they may be used to minimize or prevent disturbances in one portion of a circuit that might otherwise be caused by normal-mode or common-mode signals in another part of a circuit.
Digital isolators are signal isolators that are optimized for use in transferring digital signals across an isolation barrier. Maintaining digital signal integrity may require that a digital isolator exhibit wide bandwidth to enable transfer of high frequency digital signals with fast rise and fall times, and may also require small input-to-output propagation delay times so that the relative timing of signal transitions on the output side of the isolator closely conforms to the timing of the transitions at the input. Analog isolators are signal isolators that have the ability to transfer analog signals across an isolation boundary. Maintaining analog signal integrity may additionally require that an analog isolator exhibit linearity over a wide bandwidth so that the relative amplitude of the signal on the output side of the isolator closely conforms to the relative amplitude of the signal at the input.
Some isolators use an isolation transformer, e.g. a pair of magnetically coupled insulated windings, to provide the galvanic isolation between the input and output ports. The transformer may be coreless. Analog Devices, Norwood, Mass., USA, for example offers an iCoupler™ product family of digital isolators using chip-scale transformer windings isolated by means of thick polyimide insulation and CMOS integrated circuits that are packaged with, but separate from, the chip-scale transformer, to provide input and output interfaces. Operation of iCoupler products requires user provided primary-side and secondary-side bias-supplies for powering the input and output interface circuitry within the isolator.
Use of a combination of magnetic and copper shields is described in Tang et al, Evaluation of the Shielding Effects on Printed-Circuit-Board Transformers Using Ferrite Plates and Copper Sheets, IEEE Transactions on Power Technology, Vol. 17, No. 6, November 2002, pp. 1080-1088. A multilayer coreless inductor with copper shields is described in Lee et al, Multilayer Stacked Coreless Printed Spiral Winding Inductor with Wide Frequency Bandwidth, IEEE Energy Conversion Congress and Exposition, 2009, pp. 1002-1009. A transformer having multiple winding layers is described in Kawanobe, Transformer, U.S. Patent Application Publication No. US 2003/0095026 A1.
Transfer of digital data across a magnetically isolated barrier using a modulated high frequency carrier is described in Mohan et al, Power Electronics—Converters, Applications and Design, 2nd Edition, John Wiley and Sons, 1995, pp. 708-709; Haigh et al, Isolator for Transmitting Logic Signals Across an Isolation Barrier, U.S. Pat. No. 6,262,600; and Hui et al, Optimal Operation of Coreless PCB Gate Drive Circuits, IEEE Transactions on Power Technology, Vol. 14, No. 3, May 1999. Output circuitry powered by energy transferred across the transformer is disclosed in Mohan, supra, and Hui, supra.